Carbon-carbon engine components and method of fabrication

ABSTRACT

An internal combustion engine component assembly including piston, wrist pin, and cylinder sleeve, all constructed of a matching carbon-carbon composite is disclosed. The piston is a two-piece assembly divided in crown and skirt, each fabricated individually to optimize the most desirable properties in the respective cylinder areas in which they operate. The crown is fabricated by placing the fiber and binder into a compression mold and pyrolizing(heating) the resulting preform at a high temperature in the range of 1500 to 2000 degrees C. to achieve high temperature strength, and high thermal conductivity that continue after machining to the finished crown part. The skirt, a separate piece, on the other hand is fabricated differently to seek higher lubricity and better wear resistance along the cylinder wall with lower thermal conductivity to minimize heat loss. The skirt preform is fabricated by wrapping the fiber around a mandrel and subsequently heating and pyrolizing at temperatures far less than the heat treatment temperature of the crown preform resulting in significant time and cost saving. The skirt precursor is machined and then assembled to the crown to complete the piston. The piston skirt, the wrist pin and the cylinder sleeve are also fabricated using similar composition matching techniques to minimize tolerances between these parts. All three of these parts are separately preformed on mandrels utilizing the same wrapping angles to equate wrap strength. Then, the resulting preforms of each are preheated and pyrolized at the same temperatures to almost similar elevated temperature strength and, most importantly, coefficients of thermal expansion. This technique permits the engine designer to reduce the clearance between these parts, thereby minimize blow by, reduce lubrication requirements, and increase engine horsepower.

RELATED APPLICATION

This application is a Division of my U.S. Ser. No. 09/070,325, Filed:Apr. 30, 1998, entitled “CARBON-CARBON ENGINE COMPONENTS AND METHOD OFFABRICATION”, and it is being filed under the provisions of 35 U.S.C.121, now U.S. Pat. No 6,029,346 on Feb. 29, 2000.

BACKGROUND OF THE INVENTION

The present invention relates to the fabrication of internal combustionengine parts from carbon-carbon composites. Carbon-carbon composites aremade of carbon fiber reinforced in carbon matrix.

In the past, carbon-carbon parts have been used primarily inaeronautical and space applications because of their light-weight andhigh temperature properties. However, these characteristics are alsoextremely beneficial in industrial and automotive engines as evidencedby the fact that aluminum pistons(density 2.7 g/cm³), which are alsolighter than steel(8.0 g/cm³), but heavier than carbon-carbon (1.7g/cm³), have achieved significant commercial success in these markets.However, aluminum has a number of disadvantages. The relative differencein thermal strength and coefficient of thermal expansion of the aluminumpistons with other mating engine components, require large clearancesbetween the piston and the adjacent walls to eliminate interference andgalling between the piston and the cylinder wall and the wrist pin. Toimprove engine efficiency, piston rings are used in these aluminumpistons to seal the gap between the piston and the cylinder wall. Infact, multiple rings with staggering gaps are required to prevent highpressure leakage and possible piston erosion from local high flow ratesat the rings, piston and cylinder wall inner face. Because of the poor,high temperature strength of aluminum, it has been found necessary tolower the piston rings from the crown to prevent the rings from stickingin the ring grooves, and this has resulted in unburned hydrocarbonbuild-up in the space around the piston above the ring yielding reducedengine efficiency, noting that aluminum melts at 660 degrees C. (and themaximum application temperature is 300 degrees C.), which is well belowthe typical combustion engine temperature. Also, large amounts oflubricant are required to reduce the piston and cylinder walltemperature and wear rates in aluminum piston assemblies.

The carbon-carbon components are desirable in this environment becauseof their resistance to high temperature and thermal shocks, coupled withhigh temperature strength. In some cases, the carbon-carbon piston caneliminate the necessity of piston rings because of the negligiblecoefficient of thermal expansion of carbon-carbon(1-2 ppm), which is farless than aluminum(18-20 ppm). Even at high temperatures, thecarbon-carbon parts uniquely maintain strength, allowing the piston tooperate at both higher temperature and higher pressure than metalpistons. Thermal efficiency of the engine is also improved because ofthe high emittance and low thermal efficiency of carbon-carbon,resulting in less heat loss into the piston and the cooling system.

The carbon fibers in the carbon-carbon composite are known asprecursors, and there are three different types; namely, rayon,polyacrylonitrile, and pitch. Rayon has been largely abandoned in recentyears because of the resulting poor quality fibers so that today fibersare predominantly made from P.A.N.(polyacrylonitrile) or pitch. P.A.N.is preferred for high strength, whereas pitch derivatives are desirablefor high modulus and high thermal conductivity.

In reality, however, the use of carbon-carbon composites in enginecomponents in the industrial and automotive market has not beenextensive primarily for two reasons. The first is cost. In the early1990's, carbon fiber used to cost about $40/lb., and now costs$8-$9/lb., and the near term projections are for under $5/lb. This costreduction and an increased demand for fibers, which is projected, shoulddrive the fiber cost down further making the carbon-carbon composites avery strong engineering material to replace steel and aluminum in manyapplications.

The second reason why carbon-carbon composites have not achieved greatcommercial success is the inability of fabricators to optimize andreduce the cost of the fabrication process. This is due in part to thedifficulties in processing techniques to convert the binder to completecarbon which can hold the fibers, so the fibers therein reinforce thebinder in such a way to have suitable engineering properties.Traditional processing consists of mixing the fiber with resin andpreform into the desired shape. These preforms are kept in a hightemperature furnace and heat treated for several hours ranging from 800to 2000 degrees C. After firing, the composites are placed in a CVDfurnace and densified. CVD refers to chemical vapor deposition. Due tothe nature of CVD, it is extremely difficult to fabricate thickspecimens with uniform density. As such, even for thin samples the CVDprocess takes from a few days to several weeks to finish. The CVD issometimes replaced by chemical vapor infiltration (CVI), which causescarbon to close on the outside walls of the preform and inhibitpenetration to the inside walls. Thus, in addition to time costs, theresulting crusting problem and its removal made these processes highlylabor intensive and not conducive to high volume production.

In CVD, hydrocarbon gas is sent through the preform to crack it withhigh heat. This breaks the carbon down from hydrogen.

Another deficiency in the prior art of carbon-carbon high temperaturecomponents is the failure to optimize the performance of the componentby controlling and varying the performance characteristics at specificlocations on each part. For example, a rotating shaft under load willrun hotter in the area of the bearings than at a point midway betweenthe bearings. Prior art methodology for constructing such a shaft wouldresult in homogenous physical properties throughout the shaft, and it isthis approach that has in the past contributed to the high cost and lessthan optimum performance for carbon-carbon composites.

The Taylor, U.S. Pat. No. 4,683,809, assigned to NASA, shows alight-weight carbon-carbon piston with no piston rings. The piston isconstructed in one piece and the fibers are laid up randomly throughoutthe piston. The methodology of fiber lay-up tends to disburse the fibersrandomly resulting in internal cracks, unreliability, and low strength.The resulting piston component is heavy with poor fracture toughness.Taylor also suggests in this patent a carbon-carbon cylinder wall 60,but is silent as to how the cylinder wall or sleeve is formed or how itsperformance optimized.

Another Taylor patent, also assigned to NASA, is U.S. Pat. No.4,736,676, which discloses a composite piston structure including acarbon-carbon or ceramic piston cap 11 with a metallic piston body 13.This piston is quite complicated and too difficult to manufacture incommercial production.

A later Taylor, et al., U.S. Pat. No. 4,909,133, also assigned to NASA,discloses a carbon-carbon piston that has a tubular closed ended knittedpreformed sock of carbon fibers 11 imbedded within the matrix of thepiston structure on the piston crown side wall in the inside surface.

Finally, the Fluga, U.S. Pat. No. 5,154,109, discloses a method ofmanufacturing a piston and piston rod in which a layer of carbon fibersis triaxially braided on a mandrel with a cylindrical body. A secondlayer of carbon fibers is triaxially braided over the first layer. Thefiber layers are spaced from one another and impregnated with a thermoset resin. The preform is unidirectional in the sense that it does nothave a uniform axial diameter. These are extremely difficult tomanufacture and difficult to densify using CVD. Furthermore, the designis not flexible because the whole structure is made of one type ofmaterial, and thermal expansion is difficult to predict, and in somecases, may expand obliquely.

It is the primary object of the present invention to ameliorate theproblems noted above and provide an improved method and structure forfabricating carbon-carbon light-weight components intended for hightemperature environments.

SUMMARY OF THE PRESENT INVENTION

In accordance with the present invention, specific internal combustioncomponents including piston, wrist pin, and sleeve are constructed ofmatching carbon-carbon composites. The piston is a two-piece assemblydivided in crown and skirt, each fabricated to optimize the mostdesirable properties into different cylinder areas in which theyoperate. The crown is fabricated by placing carbon fiber and carbonmatrix into a compression mold and pyrolizing(heating) the resultingpreform at a high temperature in the range of 1500 to 2000 degrees C. toachieve high temperature strength and high thermal conductivity thatcontinue after machining to the finished crown part. The skirt, aseparate piece, on the other hand is fabricated differently to seekhigher lubricity along the crown wall with lower thermal conductivityand lower high temperature strength. The skirt preform is fabricated bywrapping fiber or fabric around a mandrel followed by successive stagesof heating and pyrolizing at temperatures substantially less than thecracking temperature of the crown preform resulting in substantial timeand labor savings. The skirt preform is machined and then assembled tothe crown to complete the piston.

The piston skirt, the wrist pin, and the cylinder sleeve are alsofabricated using a matching fabrication technique to minimize tolerancesbetween these parts. The three parts are separately preformed onmandrels utilizing the same wrapping angles to achieve matchingproperties. Then, the resulting preforms of each are preheated andpyrolized at the same temperatures to almost equate the elevatedtemperature strength and most importantly, coefficients of thermalexpansion. This technique permits the engine designer to reduce theclearances between these parts, minimize blow by, reduce lubricationrequirements, and increase horsepower.

According to the present invention and based on a differential analysisof piston environment, the property requirements for the skirt are highself-lubricious properties and improved wear resistance while for thecrown high thermal strength and high thermal conductivity. To achievethis, the crown is fabricated in a compression mold separately from theskirt. This permits the piston designer to select the fiber orientationin the preform for the crown and the subsequent heat treating,pyrolizing and CVD steps to best accommodate the temperature andpressure conditions above the crown. Usually, this dictates a pyrolizingtemperature of about 2000 degrees C.

The skirt, on the other hand, is manufactured by wrapping fibers on amandrel with the fiber layers running along different directions.Building the skirt on a mandrel enables a hybrid of fiber additions,such as stabilized P.A.N., low temperature pitch or a high temperaturepitch fiber in such a way to tailor the properties for specificstrength, thermal conductivity, and thermal expansion, etc. This usuallydictates a significantly lower heat treating temperature in the range of600 to 800 degrees C. After machining the crown and skirt, they arethreaded to one another and the threads welded permanently in a heattreating furnace at 400 degrees C.

This method of fabrication has several advantages. One, it enablestailorable piston properties at different parts of the piston to achievesuperior performance characteristics which is otherwise not able toachieve from prior art. Other significant advantages are productionvolume is significantly increased due to the smaller preforms makingheat treatment and C.V.D. more efficient and less time-consuming. Thelower temperature skirt curing also incrementally increases productionefficiency. Separation of crown and skirt also enables the manufacturerto mix and match skirts and crowns to increase assembled partvariations. A still further advantage is that this method of fabricationlends itself more to automation than prior techniques.

A binder is also used for the threads between the crown and the skirt,which may be pitch, phenolic, or any high temperature resin.

Other objects and advantages of the present invention will be moreclearly apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sub-assembly of engine components according to the presentinvention including piston, wrist pin and cylinder sleeve;

FIG. 2 is a sub-assembly cross-section of the piston crown illustratedin FIG. 1;

FIG. 3 is a right side view of the piston crown illustrated in FIG. 2;

FIG. 4 is a left side view of the piston skirt illustrated in FIG. 5;

FIG. 5 is a sub-assembly longitudinal section of the piston skirtillustrated in FIGS. 1 and 4;

FIG. 6 is a partly fragmented sub-assembly of the piston skirtillustrated in FIGS. 1, 4 and 5, rotated 90 degrees in two orthogonalplanes from the FIG. 5 view;

FIG. 7 is a sub-assembly of the piston wrist pin;

FIG. 8 is a perspective view of the cylinder sleeve illustrated in FIG.1;

FIG. 9 is an exploded view of the layers in the crown preform;

FIG. 10 is an exploded view of a compression mold forming the crownpreform;

FIG. 11 is an end view of a mandrel upon which the carbon fibers arelayered to form the piston skirt and wrist pin;

FIG. 12 is a perspective view of the mandrel wrapping illustrated inFIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings and particularly FIG. 1, a partial piston andcylinder assembly 10 is illustrated including a carbon-carbon cylindersleeve 11, and a piston assembly 12 consisting of a carbon-carbon crown14 threaded into a carbon-carbon skirt 16 carrying a carbon-carbon wristpin 18. The crown 14 has a domed upper surface 19, a groove 20 thatdefine with top surface 21 of the skirt 16, a groove for a piston ring23. A second piston ring 24 is received in annular groove 25 near thetop of the skirt 16. The rings 23 and 24 and the grooves 20 and 25 arepositioned close to the top of the piston to reduce blow by and unburnedhydrocarbons around the periphery of the piston, made possible by thehigh thermal conductivity and high heat strength of the crown 14.

The crown has a depending annular flange 27 that is threaded at 28received in a threaded annular counter bore 30 in the top of the skirt16. Skirt 16 has an enlarged through bore 32 to reduce skirt weight.Skirt 16 also has a cross recess 34, as seen in 66, in its lower surfacethat also contributes to weight reduction and port relief in someapplications.

Skirt 16 has a cross bore 35 there-through for receiving the wrist pin18, and it has spaced annular grooves 37 and 38 therein for receivingsnap rings 40 for holding the wrist pin in position. Snap rings 40 fitinto spaced annular recesses 42 in the wrist pin 18, as seen in FIG. 7.

The processes for fabricating the sleeve 11, the piston 12, and thewrist pin 18, will be described as follows in conjunction with FIGS. 9,10, 11 and 12.

The preform for the crown 14 is fabricated in a compression mold 50having an upper pressure platen 51. In one method to achieve highthermal conductivity and high temperature strength, the carbon fibersare layered with a pitch matrix in mold 50, as indicated by layers 52,53, 54, 55, etc. The fibers in each layer are orthogonally related tothe fibers in the adjacent layers.

Alternatively, the mold 50 can be filled with chopped fibers in a pitchmatrix in any orientation desired. It should be understood that thefibers in the crown preform 56 are generally perpendicular to the axisof the final piston assembly 12. The fiber orientation in the preform 56can be tailored for a particular engine; for example, the crown fiberorientation for gasoline engines are positioned for high thermalconductivity and for lower conductivity in diesel applications. Thecrown preform 56 is heated in a compression molding machine associatedwith mold 50 to about 600 degrees C. The preform 56 is thereaftercracked by pyrolization in a furnace at 1500 to 2000 degrees C.

The crown is then machined from the preform 56 utilizing conventionalmachining techniques.

The piston parts, as well as the liner 11 and the wrist pin 18, can becoated if desired based on requirements to increase their performancecharacteristics by a CVD of carbon, SiC, Si₃ N4, or by electrolyticdeposition of metals and their compounds such as Ni or Cu or by sol geland other commercially available coatings.

Preforms for the cylinder sleeve 11, the skirt 16, and the wrist pin 18,are formed by wrapping on a mandrel, such as mandrel 60, illustrated inFIGS. 11 and 12.

In one method illustrated in FIGS. 11 and 12, a fiber sheet 61 isdirected to the mandrel at +45 degrees while fiber sheet 62 is directedat the mandrel at −45 degrees, and fiber sheet 63 is directed at themandrel at 0 degrees. The fibers in each layer or sheet runlongitudinally in the sheets as shown.

Building the sleeve preforms from a bobbin-like mandrel enables a hybridof fiber additions, such as stabilized PAN, PAN, low temperature pitchor high temperature pitch fiber in such a way to tailor the propertiesfor specific strength, thermal conductivity, thermal expansion, etc.Chopped fibers may also be used with the mandrel technique by applyingthe chopped fibers uniformly to sheeting.

This mandrel process can be done in a machine similar to a lathe so thatthe sleeves are fabricated uniformly and rapidly with consistentquality. The resulting preform 65 can be preheated if desired on themandrel 60 to stretch the fibers in the preform, which increases highheat strength for the resulting part.

Thereafter, the preforms are placed in a furnace and heated at 600 to800 degrees C. for the final cure removed from the mandrel 60.

The cylinder sleeve 11 and the skirt 16 have their preforms 65 made inexactly the same way, and are cured for the same time at the sametemperature to achieve the optimum matching of piston skirt to cylindersleeve. This technique minimizes the required piston-cylinder clearanceby minimizing differential thermal growth between the piston and thecylinder sleeve.

The piston is assembled by threading the crown 14 to the skirt 16 with abinder such as pitch, phenolic or any high temperature resin. Theseparts are subsequently heat treated in the range of 400 to 1000 degreesC. depending upon the nature of the binder and the requirements.Subsequent to this assembly, the piston is further machined with wristpin bore 35 and grooves 20, 25. A separate heat treatment of the skirtresults in a cost reduction because is is no longer required to be madeto the high temperature requirements of the crown. Because the piston ismade in two parts, the smaller preforms are simpler and faster to heattreat and CVD, if necessary. Also as noted above, different crowns anddifferent skirts can be interchanged to provide a variety of productshaving different characteristics for different applications; i.e., acustom assembly system. It is also easier to create combustion bowl andvalve relief on the piston crowns because of this fabrication method.

Furthermore, the piston rings 23 and 24 can be moved upwardly comparedto aluminum piston technology for reduced blow by improved ring pack,and reduced clearance volume. This carbon-carbon assembly enables theengine to run lean, and as a result of the reduced weight, considerablehorsepower will be realized with improved thermal efficiency, improvedfuel consumption, and reduced emissions.

What is claimed is:
 1. A carbon-carbon piston for an internal combustionengine, comprising: a piston having an axis and including a piston crownconstructed from a first preform of carbon fiber and binder havingproperties of thermal conductivity and strength to meet the cylinderconditions above the piston, and a separate skirt constructed from asecond separate preform of carbon fiber and binder having properties ofthermal expansion and thermal conductivity desirable on the side of thepiston, said skirt being a cylinder sealing skirt and having an outerdiameter at least as great as the outer diameter of the piston crown,and means for reducing the outer thermal expansion of the skirtincluding carbon fiber in the skirt located to restrict radial thermalexpansion.
 2. A carbon-carbon piston as defined in claim 1, wherein thecrown preform has layered fibers with each layer being orthogonallyrelated to the adjacent layer with the layers oriented perpendicular tothe axis of the resulting piston for higher strength.
 3. A carbon-carbonpiston as defined in claim 1, wherein the crown preform has choppedfibers for better molding and isotropic properties.
 4. A carbon-carbonpiston as defined in claim 1, wherein the skirt preform has its fibersoriented in an annular direction relative to the axis of the piston. 5.A carbon-carbon piston as defined in claim 4, wherein the skirt preformhas layered fibers with the layers being angular relative to the axis ofthe resulting piston.
 6. A carbon-carbon piston as defined in claim 1,wherein the piston crown has a central downwardly depending threadedannular wall, said skirt having an upper central threaded bore intowhich the crown annular wall is threaded.
 7. A carbon-carbon piston asdefined in claim 1, wherein the skirt fibers are angularly related tothe axis of the resulting piston.
 8. A carbon-carbon piston for aninternal combustion engine, comprising: a piston having an axis andincluding a piston crown constructed from a first preform of carbonfiber and binder, and a separate skirt constructed from a secondseparate preform of carbon fiber and binder, said crown preform havinglayered fibers with each layer being orthogonally related to theadjacent layer with the layers oriented perpendicular to the axis of theresulting piston, said skirt being a cylinder sealing skirt and havingan outer diameter at least as great as the outer diameter of the pistoncrown, and means for reducing the outer thermal expansion of the skirtincluding carbon fiber in the skirt located to restrict radial thermalexpansion, said skirt fibers extending in an annular direction relativeto the axis of the piston.
 9. A carbon-carbon piston as defined in claim8, wherein the skirt has its fibers oriented in an annular direction tothe axis of the piston and including a plurality of wound layers.
 10. Acarbon-carbon piston as defined in claim 7, wherein the piston crown hasa central downwardly depending threaded annular wall, said skirt havingan upper central threaded bore into which the crown annular wall isthreaded.
 11. A carbon-carbon piston for an internal combustion engine,comprising: a piston having an axis and including a piston crownconstructed from a first preform of carbon fiber and binder, and aseparate skirt constructed from a second separate preform of carbonfiber and binder, said skirt preform has its fibers oriented in adirection generally perpendicular to the axis of the piston andincluding a plurality of wound layers, said skirt being a cylindersealing skirt and having an outer diameter at least as great as theouter diameter of the piston crown, and means for reducing the outerthermal expansion of the skirt including carbon fiber in the skirtlocated to restrict radial thermal expansion, said skirt fibersextending in an annular direction relative to the axis of the piston.12. A carbon-carbon piston as defined in claim 11, wherein the crownpreform has layered fibers with each layer being orthogonally related tothe adjacent layer with the layers oriented perpendicular to the axis ofthe resulting piston for higher strength.
 13. A carbon-carbon piston asdefined in claim 11, wherein the crown preform has chopped fibers forbetter molding and isotropic properties.
 14. A carbon-carbon piston asdefined in claim 11, wherein the piston crown has a central downwardlydepending threaded annular wall, said skirt having an upper centralthreaded bore into which the crown annular wall is threaded.